Ways to Save Energy in Pumps

Here is my experience based on energy audits of pumping systems in various chemical, metal, textile & petrochemical units.

• Design systems with lower capacity and total head

1. Do not assume these requirements are fixed.
2. Calculate flow requirement based on actual mathematical nos without margins in each stage & then add 10-20% straightforward as Normal capacity of the pump. For example if process side heat load in an exchanger is based on normal flow of say 100 M3/hr then do not consider cooling water requirement for peak condition of 120 or say 140 M3/hr. Just calculate it based on normal flow of 100 M3/hr at this stage.
3. Total head requirements can be reduced by: lowering process static gage, pressure, minimizing elevation rise from suction tank to discharge tank, reducing static elevation change by use of siphons, lowering spray nozzle velocities, lowering friction losses through use of larger pipes and low-loss fittings, and eliminating throttle valves.
4. After calculating total requirement of Flow & Head this way, simply add 10-20% in both parameters as design margin based on your judgement about process variation. This should be your normal capacity. You still have higher margins becasue rated condition are further higher than these values.
5. Also keep in mind that worst conditions dont come all simultaneously. You can still meet few peak demands.
6. Dont worry about undersizing of your pump. You can add later & this approach is beneficial in overall longer run as you can switch your additional capacity ON & OFF.
7. This can give you a saving of ~10-20% in your pumping system. A thorough review is must.

• Emphasize on Efficiency first

1. Despite the tendency to emphasize initial cost, you will save cost in the long run by selecting the most efficient pump type and size at the onset.
2. The choice of a pump depends on the service needed from the pump. Considerations are flow and head requirements, inlet pressure or net positive suction head available, and the type of liquid to be pumped.
3. Maximum attainable efficiency of a centrifugal pump is influenced by the designer's selection of pump rotating speed as it relates to "specific speed." Purchasers need to be aware of this, as well as the decision criteria for determining the type of pump to use.
4. Consider LCC (Life Cycle Costing) option instead of initial cost only. Click here to learn more about LCC Analysis.
5. Remember ENERGY is the most expensive "commodity" today.
6. People generally loose ~80% more money due to non LCC approachover a period of its service life.

• Divided Use

1. Design or select no of pumps based on different possible scenarios & always follow the operation philosophy of bulk & makeup supply for any system.
2. This approach saves at least 10-15% over conventional selection of equal size pumps.
3. This helps in putting the smaller pump on auto mode with header pressure switch so that excess pump capacity can be turned on/off.
4. Two pumps can be operated in parallel during peak demand periods, with one pump operating by itself during lower demand periods. Energy savings result from running each pump at a more efficient operating point and avoiding the need to throttle a large pump during low demand.
5. Analternative is to use one variable-speed pump and one constant-speed pump. Use or selection depends on the process behaviour e.g. how fast the demand is changing? How many time it is changing? Is my process critical? etc.

• Avoid end of curve operation

1. Generally in case of cooling water pumps head & flow both are selected with plenty of margins, comfortable to the cushion needed by selection manager. This results in near end curve operation without throttling. This is worst operation of pumps in almost every situation. Avoid it.

• Use pumps as drives

1. Use them as drivers / turbines to recover pressure energy that would otherwise be wasted.
2. Practically all centrifugal pumps will perform as turbines when operated in reverse.
3. A hydraulic power recovery turbine can recover pressure energy when used to drive a generator, or assist the driver of a pump or a compressor.